Apparatus, systems and methods utilizing adjacent-channel power dependent automatic gain control for digital television demodulation

ABSTRACT

An embodiment of the present invention provides a method for digital television demodulation, comprising using adjacent-channel power dependent automatic gain control (AGC) for the digital television demodulation, wherein an AGC technique takes into account a total power as well as power of adjacent channels to control gain of a gain control amplifier.

BACKGROUND

In digital TV, the power levels of channels that are adjacent to thewanted channel can be 30 dB to 40 dB higher than the wanted channel.Part of this power is removed by the tuner, yet a significant part ofthis appears at the input of the analog to digital (A/D) converter (ADC)of the digital demodulator. The automatic gain control circuit has toadjust the power level of this composite (wanted+adjacent) signal at theinput of the ADC to get optimum receiver performance.

In emerging DTV standards, a channel may be time-multiplexed betweendifferent services. For example, in DVB-T2 there are Future ExtensionFrames (FEFs) which are intended to carry other services. Although notlimited in this respect, one such service already identified is thesignaling for transmitter identification. The power levels of differentservices may not be the same.

Hence in DVB-T2, the power level of FEFs may well be different from thatof other frames. Although this is not a problem with current systems, itis a problem that is likely to occur in the future when DVB-T2 typetransmissions with multiple services are widely adopted.

Thus, a critical need is prevalent for apparatus, systems and methodsthat enable DTV demodulators to overcome this potential problem.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject matter regarded as the invention is particularly pointed outand distinctly claimed in the concluding portion of the specification.The invention, however, both as to organization and method of operation,together with objects, features, and advantages thereof, may best beunderstood by reference to the following detailed description when readwith the accompanying drawings in which:

FIG. 1 illustrates a conventional AGC Control Architecture; and

FIG. 2 shows a new AGC Control Architecture according to embodiments ofthe present invention.

It will be appreciated that for simplicity and clarity of illustration,elements illustrated in the figures have not necessarily been drawn toscale. For example, the dimensions of some of the elements areexaggerated relative to other elements for clarity. Further, whereconsidered appropriate, reference numerals have been repeated among thefigures to indicate corresponding or analogous elements.

DETAILED DESCRIPTION

In the following detailed description, numerous specific details are setforth in order to provide a thorough understanding of the invention.However, it will be understood by those skilled in the art that thepreset invention may be practiced without these specific details. Inother instances, well-known methods, procedures, components and circuitshave not been described in detail so as not to obscure the presentinvention.

Although embodiments of the invention are not limited in this regard,discussions utilizing terms such as, for example, “processing,”“computing,” “calculating,” “determining,” “establishing”, “analyzing”,“checking”, or the like, may refer to operation(s) and/or process(es) ofa computer, a computing platform, a computing system, or otherelectronic computing device, that manipulate and/or transform datarepresented as physical (e.g., electronic) quantities within thecomputer's registers and/or memories into other data similarlyrepresented as physical quantities within the computer's registersand/or memories or other information storage medium that may storeinstructions to perform operations and/or processes.

Although embodiments of the invention are not limited in this regard,the terms “plurality” and “a plurality” as used herein may include, forexample, “multiple” or “two or more”. The terms “plurality” or “aplurality” may be used throughout the specification to describe two ormore components, devices, elements, units, parameters, or the like. Forexample, “a plurality of stations” may include two or more stations.

All known automatic gain control (AGC) circuits in digital television(DTV) receivers adopt the approach given in FIG. 1, because in currentDTV systems the ACI power levels do not change that frequently. Powerlevels may change slowly over long time periods and then the AGCcircuits adapts for these changes. In future systems employing standardslike DVB-T2 with multiple services, the ACI power levels can changequite frequently because of Future Extension Frames (FEFs) allocated forother services. In such systems if the conventional method in FIG. 1 isadopted, the AGC will amplitude modulate the wanted channel and therebyreduce performance.

Embodiments of the present invention provide AGC techniques, includingbut not limited to an AGC circuit, that takes into account the totalpower as well as the power of the adjacent channels (ACT power) tocontrol the gain of the gain control amplifier. This enables DTVreceivers of embodiments of the present invention to work in an optimumway in any future DVB-T2 based system. Although embodiments of thepresent invention are illustrated herein using DVB-T2, the presentinvention is applicable to other Digital TV systems as well and thus thepresent invention is not limited to DVB-T2.

Assume that the wanted channel is a DVB-T (or even a DVB-T2) channel.One may consider a scenario in which a DVB-T2 channel with multipleservices (implemented via FEF) happens to be adjacent to the wantedchannel. The power level in the adjacent channel will then varyfrequently with time and hence the total power at the input to the ADCwill also vary unpredictably with time. The AGC will attempt to keepthis total power level at the specified target value by adjustingamplifier gain. In doing so it will inadvertently amplitude modulate thewanted channel, leading to loss of performance.

Looking now at the figures, FIG. 1, shown generally as 100, shows theautomatic gain control circuit of a conventional receiver. Tuner isshown at 105 which is output to Gain Control Amplifier 110, whichprovides input for analogue to digital converter (ADC). Power level ismeasured at 130 at the output of the ADC 115 and adjusts the gain 135 ofthe amplifier 110 preceding the ADC to give optimum ADC performance andproviding input to channel filter 120, the output of which is sent todemodulator 125.

FIG. 2 at 200, illustrates the new automatic gain control circuitprovided in embodiments of the present invention. It may have twoinputs. A first input is a power level 235 at the output of the ADC,which may be similar to conventional AGC circuits. A second input 230,distinct from previous art and unique to embodiments of the presentinvention, is the power level of the adjacent channels only. The outputof AGC 135 is used to control the gain of gain control amplifier 210. At240 ACI power rations are computed.

The operation of the conventional AGC circuit is known to those ofordinary skill in the art and is thus not described in any significantdetail herein, but may comprise the following main components: a circuitfor working out the difference (or error) between the received power andthe target power level, an adjustable gain to be applied to this errorto control the loop bandwidth or response time, a logarithmicnon-linearity and a loop filter comprising an accumulator (integrator).

In many applications, the amplitude probability density function may beassumed Gaussian (via Central Limit Theorem). Then setting the standarddeviation (i.e. square-root of the target) to about 25% of the peak ADCvalue will ensure a clipping probability of about 1e−5 which may beconsidered acceptable for most digital TV (DTV) applications.

A key new feature of the new AGC provided in embodiments of the presentinvention is that it responds to the composite power as well as to theresidual power of the adjacent channel. In DTV applications the adjacentchannel power may be +40 dB and the filters 220 in the tuner often donot provide stop band attenuation greater than 40 dB. Hence the residualpower of the Adjacent Channel Interference (ACI) seen by the demodulatorADC 215 may well be equal to or higher than the wanted channel. Thereare also situations in which a fixed-bandwidth (8 MHz) tuner 205 has tosupport 6, 7 and 8 MHz bandwidth DTV. Then for 6 and 7 MHz DTV the ACIpower seen by the demodulator 225 will be much higher than that of thewanted channel. There is a digital channel filter 220 in the digitalfront-end of the demodulator 225 to filter out the ACI and to isolatethe wanted channel.

Assume that the adjacent channel is a DVB-T2 channel. The DVB-T2standard allows non DVB-T2 frames to be interspersed within the DVB-T2frame structure. These are called Future Extension Frames (FEFs) and areintended for use by other services in the future. One such servicealready planned is transmitter identification. It is likely that in thefuture DVB-T2 channels will be time-shared with other services throughthe use of FEFs.

There is no guarantee that the power of FEF will be the same as anyother DVB-T2 frame in the same transmission. If the FEF is part of thewanted channel, then the receiver will precisely know the location ofthis from signaling parameters and hence can easily take preventativeaction. However, this is not the case if the DVB-T2 transmission withFEFs happens to be an adjacent channel. Then there could be significantand relatively frequent fluctuations in the total power (wanted plusadjacent channel power). The conventional AGC circuit in FIG. 1 willrespond to this total power and try to keep this total power at thetarget level. To do this it will adjust the gain of the amplifier 110shown in FIG. 1. The net effect of this is that the wanted channel willbe subjected to undesirable amplitude changes due to the fluctuations ofthe power levels of adjacent channels.

Embodiments of the present invention provide an algorithm shown in FIG.2 wherein the present invention taps off the input 217 and the output223 of the above mentioned digital channel filter 220 and use these tomeasure the residual power of the adjacent channels. The power at theinput 217 of the channel filter 220 is the total power, which is the sumof the wanted and ACI power. The power at the output of the channelfilter 220 is the wanted power level only. Hence the ACI_Power_Ratio isgiven by the following equation:

$\begin{matrix}{{{ACI\_ Power}{\_ Ratio}} = {\frac{{Channel\_ Filter}{\_ Input}{\_ Power}}{{Channel\_ Filter}{\_ Output}{\_ Power}} - 1}} & (1)\end{matrix}$

Two values of ACI_Power_Ratio are computed:

1. ACI_Power_Ratio_Long_Term: This is the above equation computed withchannel filter input 217 and output 223 powers averaged over arelatively long period. Furthermore, this is made to be representativeof the signal when it has “normal” ACI, i.e. no FEFs, by disabling thisaveraging during short term power fluctuations, as described below.

2. ACI Power_Ratio_Short_Term: This is the above equation computed withchannel filter input 217 and output 223 powers averaged over arelatively short period. Under normal operation, the short term ACIpower ratio will be approximately equal to the long term ACI powerratio. However, if there are DVB-T2 FEFs with higher power level thenduring these FEFs, the short term ACI power ratio will be greater thanthe long term ACI power ratio. Conversely, lower power FEFs will causethe short term ACI power ratio to be smaller than the long term one.

It is necessary to set the AGC target such that the clipping probabilityis kept below a predefined value (e.g. although not limited to, 1e−5)even in the presence of high power FEF ACI. Initially this value of highpower FEF ACI may not be known and hence the AGC target is set to a lowvalue based on a worst case assumption so that the ADC does not clipeven in the presence of the highest power ACI FEFs. This will increasethe quantization noise of the ADC and hence embodiments of the presentinvention provide a mechanism of gradually adapting the target to theobserved high-power FEF ACI levels during the tracking stage of thedemodulator to obtain the best performance from the ADC. Then, thepresent invention may detect sudden decreases or increases in the ACIpower level using the following equations:

$\begin{matrix}{\frac{{ACI\_ Power}{\_ Ratio}{\_ Short}{\_ Term}}{{ACI\_ Power}{\_ Ratio}{\_ Long}{\_ Term}} < {{Threshold\_}1}} & (2) \\{\frac{{ACI\_ Power}{\_ Ratio}{\_ Short}{\_ Term}}{{ACI\_ Power}{\_ Ratio}{\_ Long}{\_ Term}} > {{Threshold\_}2}} & (3)\end{matrix}$

Here Threshold_(—)1<1 and Threshold_(—)2>1.

If a decrease or increase of the ACI power causes a threshold crossingin accordance with the equations (2) or (3) then the AGC updates aredisabled, i.e. the AGC is frozen. Furthermore, the long-term poweraverage is frozen so that the long term averages remain unaffected byshort term changes in the ACI power level. By disabling or freezing theAGC when there are sudden changes in the ACI power level, the presentinvention reduces the extent to which the wanted signal is amplified orattenuated during these ACI FEF periods. This enables the receiver towork with minimum disruption during FEF or power fluctuation periods ofadjacent channels.

It is necessary to account for the fact that these changes in ACI powerlevel may not be caused by FEFs, but by other factors. In other words,the short term changes may well not be short term and these changes maybe there to stay. Hence if the AGC is frozen for more than a certainpre-defined time limit, then a timeout is deemed to have occurred andthe AGC is released from its from its frozen state. At the same time,the long-term average is initialized to the short-term average andallowed to adapt as normal again. An FEF has a maximum time duration of250 ms, so this gives a method of working out the above timeout period.It is understood that the present invention is not limited to anyparticular durations of FEFs.

Finally, embodiments of the present invention take into considerationautomatic AGC target adaption. It has been noted that the initial AGCtarget has been set assuming a worst case ACI FEF level. This is toensure that the ADC does not clip when this ACI FEF power level occurs.The relatively low AGC target value needed to cope with this worst casescenario could result in relatively poor ADC performance (i.e. increasedquantization noise) since the full range of the ADC is not utilized.However, this is needed to get the system into the tracking phase.

Once the system is in the tracking phase, the maximum of the actualshort term ACI power ratio is monitored over a relatively long period.The optimum AGC target setting is a function of this.AGC_Target_Optimum=f(max(ACI_Power_Ratio_Short_Term))   (4)

The above function can be derived from assuming a Gaussian probabilitydensity function for the input signal amplitude. Then the AGC targetsetting is updated to the optimum value. A step update will result in adisturbance to the DTV receiver system. Hence a gradual update in verysmall steps is made to the target over a period of time so that at theend of this period the target is optimum. The receiver continues tomonitor maximum value of short term ACI power level so that furtheradaptations can be made if there are further changes. The systemutilizing embodiments of the present invention has been simulated with acomplete transmit and receiver model and has been shown to functionwell. It has also been shown that when the present invention is notused, changes in ACI power levels due to FEFs in adjacent DVB-T2channels result in bursts of uncorrectable MPEG packets.

Embodiments of the present invention may further provide a non-volatilecomputer readable medium encoded with computer executable instructions,which when accessed, cause a machine to perform operations comprising,controlling digital television demodulation by using adjacent-channelpower dependent automatic gain control (AGC) for the digital televisiondemodulation, wherein an AGC technique takes into account a total poweras well as power of adjacent channels to control gain of a gain controlamplifier.

Also, embodiments of the present invention may provide a system,comprising a digital television transmitter, a digital televisionreceiver adapted for communication with the digital televisiontransmitter, and the digital television receiver, comprising ademodulator adapted to use adjacent-channel power dependent automaticgain control (AGC) for digital television demodulation, wherein an AGCtechnique takes into account a total power as well as power of adjacentchannels to control gain of a gain control amplifier.

While certain features of the invention have been illustrated anddescribed herein, many modifications, substitutions, changes, andequivalents may occur to those skilled in the art. It is, therefore, tobe understood that the appended claims are intended to cover all suchmodifications and changes as fall within the true spirit of theinvention.

We claim:
 1. A method for digital television demodulation, comprising:using adjacent-channel power dependent automatic gain control (AGC) forsaid digital television demodulation, wherein an AGC technique takesinto account a total power as well as power of adjacent channels tocontrol gain of a gain control amplifier; wherein said power at anoutput of a channel filter is a wanted power level only and an ACI PowerRatio is given byACI Power Ratio =(Channel_Filter_Input_Power÷Channel_Filter_Output_Power)−1; wherein two values of saidACI_Power_Ratio are computed: ACI_Power_Ratio_Long_Term, wherein saidACI_Power_Ratio computed with Channel filter input and output powers areaveraged over a long period; and ACI_Power_Ratio Short_Term, whereinsaid ACI_Power_Ratio computed with Channel filter input and outputpowers are averaged over a short period; wherein the short period isless than the long period; wherein the long period has a maximum timeduration of 250 ms.
 2. The method of claim 1, further comprising tappingoff an input and an output of a digital channel filter and using theseto measure a residual power of adjacent channels.
 3. The method of claim1, wherein power at said input of said channel filter is a total power,which is a sum of wanted and Adjacent Channel Interference (ACI) power.4. The method of claim 1, wherein said AGC responds to composite poweras well as to residual power of said adjacent channel.
 5. The method ofclaim 1 , wherein under normal operation, said short term ACI powerratio will be approximately equal to said long term ACI power ratio;however, if there are Digital Video Broadcasting (DVB-T2) FutureExtension Frames (FEFs) with higher power level then during said FEFs,said the short term ACI power ratio will be greater than said long termACI power ratio and wherein lower power FEFs will cause said short termACI power ratio to be smaller than said long term ACI power ratio. 6.The method of claim 5, further comprising setting an AGC target suchthat a clipping probability is kept below a predefined value even in apresence of a high power FEF ACI and wherein initially said predefinedvalue of high power FEF ACI may not be known and hence said AGC targetis set to a low value based on a worst case assumption so that said ADCdoes not clip even in the presence of a highest power ACI FEFs.
 7. Themethod of claim 6, further comprising detecting sudden decreases orincreases in an ACI power level using two predetermined Thresholds,Threshold_1 and Threshold_2, with Threshold_<1 and Threshold_2>1 and ifa decrease or increase of said ACI power causes a threshold crossing,then said AGC updates are disabled said long-term power average isfrozen so that said long term averages remain unaffected by short termchanges in said ACI power level, and wherein by disabling or freezingsaid AGC when there are sudden changes in said ACI power level an extentto which a wanted signal is amplified or attenuated is reduced duringsaid ACI FEF periods, thereby enabling a receiver to work with minimumdisruption during FEF or power fluctuation periods of adjacent channels.8. The method of claim 7, wherein to account for a fact that changes inACI power level may not be caused by FEFs, if said AGC is frozen formore than a pre-defined time limit, then a timeout is deemed to haveoccurred and said AGC is released from its from its frozen state andwherein at a same time, said long-term average is initialized to saidshort-term average and allowed to adapt as normal again.
 9. The methodof claim 8, further comprising using automatic AGC target adaption,wherein an initial AGC target has been set assuming a worst case ACI FEFlevel to ensure that said AGC does not clip when said ACI FEF powerlevel occurs and once in a tracking phase, a maximum of an actual shortterm ACI power ratio is monitored over a relatively long period and thensaid AGC target setting is updated to an optimum value and wherein agradual update with very small steps is made to said target over aperiod of time so that at the end of said period said target is optimumwithout a disturbance to a DTV receiver system.
 10. A digital televisionreceiver, comprising: a demodulator adapted to use adjacent-channelpower dependent automatic gain control (AGC) for digital televisiondemodulation, wherein an AGC technique takes into account a total poweras well as power of adjacent channels to control gain of a gain controlamplifier; wherein power at an input of a channel filter is a totalpower, which is a sum of wanted and Adjacent Channel Interference (ACI)power; wherein said power at an output of said channel filter is awanted power level only and an ACI Power Ratio is given byACI Power Ratio =(Channel_Filter_Input_Power ±Channel_Filter_Output_Power)−1; wherein two values of said ACI_Power_Ratio arecomputed: ACI_Power_Ratio_Long_Term, wherein said ACI_Power_Ratiocomputed with Channel filter input and output powers are averaged over along period; and ACI_Power_Ratio_Short_Term, wherein saidACI_Power_Ratio computed with Channel filter input and output powers areaveraged over a short period; wherein the short period is less than thelong period; wherein the long period has a maximum time duration of 250ms.
 11. The digital television receiver of claim 10, wherein saiddemodulator is capable of tapping off an input and an output of adigital channel filter and using these to measure a residual power ofadjacent channels.
 12. The digital television receiver of claim 10,wherein said AGC responds to composite power as well as to residualpower of said adjacent channel.
 13. The digital television receiver ofclaim 10 , wherein under normal operation said short term ACI powerratio will be approximately equal to said long term ACI power ratio;however, if there are Digital Video Broadcasting (DVB-T2) FutureExtension Frames (FEFs) with higher power level then during said FEFs,said the short term ACI power ratio will be greater than said long termACI power ratio and wherein lower power FEFs will cause said short termACI power ratio to be smaller than said long term ACI power ratio. 14.The digital television receiver of claim 13, wherein said demodulatorsets an AGC target such that a clipping probability is kept below apredefined value even in a presence of a high power FEF ACI and whereininitially said predefined value of high power FEF ACI may not be knownand hence said AGC target is set to a low value based on a worst caseassumption so that said ADC does not clip even in the presence of ahighest power ACI FEFs.
 15. The digital television receiver of claim 10,wherein said demodulator detects sudden decreases or increases in an ACIpower level using two predetermined Thresholds, Threshold_1 andThreshold_2, with Threshold_1<1 and Threshold_2>1 and if a decrease orincrease of said ACI power causes a threshold crossing, then said AGCupdates are disabled said long-term power average is frozen so that saidlong term averages remain unaffected by short term changes in said ACIpower level, and wherein by disabling or freezing said AGC when thereare sudden changes in said ACI power level an extent to which a wantedsignal is amplified or attenuated is reduced during said ACI FEFperiods, thereby enabling a receiver to work with minimum disruptionduring FEF or power fluctuation periods of adjacent channels.
 16. Thedigital television receiver of claim 15, wherein to account for a factthat changes in ACI power level may not be caused by FEFs, if said AGCis frozen for more than a pre-defined time limit, then a timeout isdeemed to have occurred and said AGC is released from its from itsfrozen state and wherein at a same time said long-term average isinitialized to said short-term average and allowed to adapt as normalagain.
 17. The digital television receiver of claim 16, wherein saiddemodulator uses automatic AGC target adaption, wherein an initial AGCtarget has been set assuming a worst case ACI FEF level to ensure thatsaid AGC does not clip when said ACI FEF power level occurs and once ina tracking phase, a maximum of an actual short term ACI power ratio ismonitored over a relatively long period and then said AGC target settingis updated to an optimum value and wherein a gradual update with verysmall steps is made to said target over a period of time so that at theend of said period said target is optimum without a disturbance to a DTVreceiver system.
 18. A non-transitory computer readable medium encodedwith computer executable instructions, which when accessed, cause amachine to perform operations comprising: controlling digital televisiondemodulation by using adjacent-channel power dependent automatic gaincontrol (AGC) for said digital television demodulation, wherein an AGCtechnique takes into account a total power as well as power of adjacentchannels to control gain of a gain control amplifier; tapping off aninput and an output of a digital channel filter and using these tomeasure a residual power of adjacent channels; wherein said power atsaid output of said digital channel filter is a wanted power level onlyand an ACI Power Ratio is given byACI Power Ratio =(Channel_Filter_Input_Power ±Channel_Filter_Output_Power)−1; wherein said ACI_Power_Ratio computed withChannel filter input and output powers are averaged over a long period;and ACI_Power_Ratio_Short_Term, wherein said ACI_Power_Ratio computedwith Channel filter input and output powers are averaged over a shortperiod; wherein the short period is less than the long period; whereinthe long period has a maximum time duration of 250 ms.
 19. Thenon-transitory computer readable medium encoded with computer executableinstructions of claim 18, wherein said AGC responds to composite poweras well as to residual power of said adjacent channel.
 20. Thenon-transitory computer readable medium encoded with computer executableinstructions of claim 19, wherein under normal operation said short termACI power ratio will be approximately equal to said long term ACI powerratio; however, if there are Digital Video Broadcasting (DVB-T2) FutureExtension Frames (FEFs) with higher power level then during said FEFs,said the short term ACI power ratio will be greater than said long termACI power ratio and wherein lower power FEFs will cause said short termACI power ratio to be smaller than said long term ACI power ratio. 21.The non-transitory computer readable medium encoded with computerexecutable instructions of claim 20, further comprising setting an AGCtarget such that a clipping probability is kept below a predefined valueeven in a presence of a high power FEF ACI and wherein initially saidpredefined value of high power FEF ACI may not be known and hence saidAGC target is set to a low value based on a worst case assumption sothat said ADC does not clip even in the presence of a highest power ACIFEFs.
 22. The non-transitory computer readable medium encoded withcomputer executable instructions of claim 21, further comprisingdetecting sudden decreases or increases in an ACI power level using twopredetermined Thresholds, Threshold_1 and Threshold_2, withThreshold_1<1 and Threshold_2>1 and if a decrease or increase of saidACI power causes a threshold crossing, then said AGC updates aredisabled said long-term power average is frozen so that said long termaverages remain unaffected by short term changes in said ACI powerlevel, and wherein by disabling or freezing said AGC when there aresudden changes in said ACI power level an extent to which a wantedsignal is amplified or attenuated is reduced during said ACI FEFperiods, thereby enabling a receiver to work with minimum disruptionduring FEF or power fluctuation periods of adjacent channels.
 23. Asystem, comprising: a digital television transmitter; a digitaltelevision receiver adapted for communication with said digitaltelevision transmitter, said digital television receiver, comprising: ademodulator adapted to use adjacent-channel power dependent automaticgain control (AGC) for digital television demodulation, wherein an AGCtechnique takes into account a total power as well as power of adjacentchannels to control gain of a gain control amplifier; wherein power atan output of a channel filter is a wanted power level only and an ACIPower Ratio is given byACI Power Ratio =(Channel_Filter_Input_Power ±Channel_Filter_Output_Power)−1; wherein two values of said ACI_Power_Ratio arecomputed: ACI_Power_Ratio_Long_Term, wherein said ACI_Power_Ratiocomputed with Channel filter input and output powers are averaged over along period; and ACI_Power_Ratio_Short_Term, wherein saidACI_Power_Ratio computed with Channel filter input and output powers areaveraged over a short period; wherein the short period is less than thelong period; wherein the long period has a maximum time duration of 250ms.
 24. The system of claim 23, wherein said demodulator is capable oftapping off an input and an output of a digital channel filter and usingthese to measure a residual power of adjacent channels.
 25. The systemof claim 24, wherein power at said input of said channel filter is atotal power, which is a sum of wanted and Adjacent Channel Interference(ACI) power.